To spawn Harkor, kill all the orcs and warboars around the keep. It's a good idea to first start the quest Orc Hunting by click the impaled skeleton at ( 5, -12, -138 ) /waypoint 5.10, -11.52, -137.59, since you have to kill all the orcs and boars for both.

Harkor rides a boar, and he a 270° "frontal" doing ranged damage: Harkor himself is rooted.

Keep all the squishies in his back quadrant and somehow avoid his red text.